when you manually manage music, you can drag and drop music and videos onto your iPhone, and still sync everything else, like contacts, calander, photos, etc... but you will not be able to sync your music and videos. this allows you to connect your iPhone into 2, 3 or even 12 other computers to move music onto the iPhone. but you can not go from the iPhone back onto the computer. that is one of the big downsides. if Apple were ever to enable disk mode, you could do this though. once you start to manually manage music and movies, you can clear everything out of your iTunes library and still keep the music on your phone. Hope this helps everyone who has been asking recently.